Newman terms this
spontaneous reasoning faculty the ratiocinative or illative faculty. And he adds that it
is actually a collection of faculties adapted to various subject matters, as may be
observed in the talents of specialists as different as Newton and Napoleon. Each of us has
this illative faculty for natural inference. In some it is a gift characterized by
precision, promptitude, and truth; in some it is natural and uncultivated, in some an
acquired habit. And it is Newman's opinion that it belongs to women more than to men. In
ordinary minds it may be degraded by bias, prejudice, passion and self-interest. The
perfection of this faculty Newman terms the illative sense, a use of the word
"sense" similar to its use in the terms "good sense", "common
sense", or "a sense of beauty". It is right judgment in ratiocination.

Newman sees the path
from inference to assent as made up of an objective logical process guided by a subjective
personal faculty for discovering truth. The illative sense is the living authority which
guides the mind in the discernment of the true limit of converging probabilities. Its role
as a guide to truth is parallel to that of conscience, our guide to what is good.

This same illative
sense is the living authority which guides the mind in the reflex act of an assent to an
assent which, in the case of a true proposition is the basis of certitude. But is there
any criterion for the accuracy of an inference in concrete matters? Yes, it is a
subjective one. The sole and final judgment on the validity of an inference in concrete
matter is committed to the illative faculty. My own illative sense or that of another is
my guide to truth in concrete matters. In no class of concrete reasonings, whether
scientific, historical, or theological, is there any ultimate test distinguishing truth
from error in our inferences, other than the trustworthiness of the sanction of the
Illative Sense.